1. Field of the Invention
The present invention relates to an equalizer used for a digital communication system, and more particularly, to a decision feedback equalizer (DFE) and a method for updating the tap coefficient of the DFE.
2. Description of the Related Art
A transmitted signal is applied to a receiving system through a channel in a digital communication system including a digital subscriber line receiving system. In a process in which the transmitted signal passes through the channel, the transmitted signal may become distorted. Accordingly, interference noise such as intersymbol interference (ISI) and narrowband interference are generated. Therefore, the equalizer is used in the receiving system in order to correctly detect the received signal. Namely, the equalizer is used for correctly restoring the received signal by compensating for distortion in the received signal when it passes through the channel.
When distortion is severe, the DFE reduces the ISI. In order for the DFE to correctly restore the received signal, an initializing process for adjusting the tap coefficients of the equalizer to a predetermined value and stabilizing the tap coefficients is essential. A method of initializing the DFE using a calibration signal and a method of initializing the DFE using a signal unknown to the transmitter and the receiver are used for initializing the DFE. The latter is called a blind equalization. In particular, since a training sequence known to the transmitter and the receiver cannot be used in a transmission system of a point-to-multipoint structure, the blind equalization is used.
FIG. 1 is a block diagram showing the structure of a decision feedback equalizer according to a conventional technology. Referring to FIG. 1, the DFE according to the conventional technology includes a feedforward equalizer 10, a feedback equalizer 12, and a slicer 14.
In general, in initializing the DFE according to the conventional technology, the tap coefficients of the feedforward equalizer 10 and the feedback equalizer 12 are not stabilized. Accordingly, a considerable amount of decision error can be generated in the slicer 14. An error signal due to a difference between an input signal of the slicer 14 and an output signal of the slicer 14 is used for updating the tap coefficients of the feedforward equalizer 10 and the feedback equalizer 12 in the DFE according to the conventional technology (not shown in FIG. 1). When the output of the slicer 14 is not correctly performed during the initialization of the DFE, the tap coefficients are updated in the wrong direction. As a result, the magnitude of the error signal increases, and the tap coefficients of the feedforward equalizer 10 and the feedback equalizer 12 diverge occasionally.
Also, since the feedforward equalizer 10 and the feedback equalizer 12 include a shift register, a multiplier, and an adder, they update the tap coefficients every symbol period. In such a case, the feedforward equalizer 10 and the feedback equalizer 12 perform many calculations at a high speed. Therefore, a large amount of current is consumed in order to operate the DFE according to the conventional technology.
It is a feature of the present invention to provide a decision feedback equalizer in which the tap coefficients of a feedforward equalizer and a feedback equalizer do not diverge but are stabilized during initialization and power efficiency is improved during a steady state.
It is another feature of the present invention to provide a method for updating the tap coefficients of the decision feedback equalizer.
According to a feature of the present invention there is provided a decision feedback equalizer, comprising a first equalizer for reducing interference noise of a channel in an input signal, a second equalizer for generating a feedback signal for reducing remaining interference noise, and an update controller for controlling the first and the second equalizers so as to freeze said second equalizer when the first equalizer is updated and to freeze the first equalizer when the second equalizer is updated.
According to another feature of the present invention, a feedforward equalizer includes a plurality of tap coefficients corresponding to a time order of a signal and removes a post-cursor of a channel as well as a pre-cursor of a channel by selecting a tap coefficient corresponding to a middle time between the tap coefficients as a main tap coefficient.
According to yet another feature of the present invention, an update value calculator is provided for calculating a tap coefficient update value of the first equalizer and a tab coefficient update value of the second equalizer as the difference between input and output signals of a slicer.
According to still yet another feature of the present invention, a first means is provided for reducing interference noise of a channel in an input signal, a second means is provided for generating a feedback signal for reducing remaining interference noise, and an update controller means is provided for controlling the first and the second means so as to freeze the second means when the first means is updated and to freeze the first means when the second means is updated.
Various other features and attendant advantages of the present invention will become more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views.